[RE-wrenches] wind load

Chris Worcester chris at solarwindworks.com
Sat Dec 5 12:19:21 PST 2009


One thing we've had to do here is stagger the connections to the rafters
from row to row thus each rafter is secured to, not every other one. This is
easy enough to do, and just adds one extra standoff or Quick Mount in your
case on the staggered row.  (we are in a high snow load area and the idea of
offsetting the connection points to the rafters on every other row is so the
snow load doesn't stress the rafters too much, spreading out the weight)

This is a win-win solution that your inspector should have no problem with.

Otherwise the truss to top plate connection might need to be addressed by
your structural engineer for its uplift value. Hopefully the home had
Simpson truss holdowns, like A-35 clips, installed during the framing, and
if this is in the original engineering that should suffice.

 

Sincerely,

Chris Worcester

Solar Wind Works
NABCEP Certified PV Installer
Phone: 530-582-4503
Fax: 530-582-4603
www.solarwindworks.com <http://www.solarwindworks.com/> 
chris at solarwindworks.com
"Proven Energy Solutions"

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
Osterberg
Sent: Saturday, December 05, 2009 8:46 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] wind load

 

Rebecca,

With three rows and six lags in every other rafter there is a tremendous
force on each of the rafters.  Unirac has excellent information in their
installation manual <http://www.unirac.com/pdf/ii227.pdf>  that shows all
the engineering info needed for ASCE-7, which is meets the  requirements for
most building departments.  If you go through the calculations, you can
determine the maximum force or load on each lag bolt.  And you should go
through the calculations to make sure that the spacing and lag bolt
penetration is adequate.  The adequacy of the roof structure is a question
for the structural engineer.  Modern truss structures will handle it, and
presumably that's what your structural engineer looked into.  At this point,
I agree with William: it is time to go over the inspector's head with the
argument that he doesn't have the authority to question the engineering.

Kent Osterberg
Blue Mountain Solar



Rebecca Lundberg wrote: 

Hi William,

Thanks for responding. We have three long rows of modules, with rails
attached every other rafter, every 4'. The inspector's argument is that two
attachments for one row might be fine, but that the wind load for the entire
array theoretically could be born by one rafter, and therefore it would pull
the rafter right out of the house. This is only my year 3 as an installer,
and while I'm not afraid to push the inspector if needed, I admit I want to
be sure he isn't right, even though an engineer says so (because I'm not an
engineer :-). So in your experience, is there a limit to an array size that
would be 'safe' for a home if the array is within the edges of the roof and
flush-mounted? I feel like that's the real argument, you might safely attach
one module, but he is saying when you multiply the modules you multiply the
wind load to the house by the same factor.

Rebecca


Message: 3
Date: Fri, 04 Dec 2009 22:21:43 -0800
From: William Miller <william at millersolar.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] PV wind load
Message-ID: <6.0.1.1.2.20091204220550.061cd348 at millersolar.com>
Content-Type: text/plain; charset="us-ascii"; Format="flowed"

Rebecca:

It sounds like you have made an air-tight case (pun intended) and you have
still not been able to convince this thick-headed inspector.  I have had a
similar experience last summer that might be relevant:  After being pushed
around beyond my patience, I fought back.  I worked up the chain of
command, stayed professional and put it all in writing.

I won the battle and set a precedent.  I have forbidden the building
department from assigning the problem inspector to my projects and, to my
amazement,  the department obeys my request!

Good luck, and let us know how it turns out

William Miller



At 05:55 PM 12/4/2009, you wrote:
>Solar is somewhat 'new technology' where I live. :-) I have a building
>inspector that feels that (32) Sanyo modules (6.88 kW array) mounted to a
>house roof is going to cause undue stress to the roof truss (it is a
>regular house in a Minnesota suburb, building height is 26', 30 degree
>roof angle, in a suburban neighborhood. 2" x 4" manufactured truss). We
>are leaving 12-24" on each edge with no PV, and are careful to attach to
>the center of each rafter.
>
>The inspector is not that concerned with dead load, but is especially
>concerned with wind load. We have provided an engineering letter that
>ascertains that a solar array mounted parallel to the roof surface is well
>within the parameters of what a typical roof truss can handle. We were
>required to have an engineer look at this address specifically, so we then
>provided an additional engineer's letter that affirmed that yes, solar
>mounted in the manner we proposed (with Quick-Mount attachment and Unirac
>SolarMount rails, all installed according to manufacturers' instructions)
>would be fine on this home. The building inspector insists that the wind
>load is excessive, and wants more analysis. Do any of you have ideas,
>experience, or data that might help me?
>
>Rebecca Lundberg

 



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